Device for forming replica images of particle distributions in a plasma stream



A nl 23, 1968 F. H. CCENSGEN ET AL 3,379,880

DEVICE FOR FORMING REPLICA IMAGES OF PARTICLE DISTRIBUTIONS IN A PLASMASTREAM Filed Feb. 24, 1965 PULSE GENERATOR VOLTAGE SOURCE INVENTORSFREDER/C H. COENSGEN BY WILLIAM F CUM/WINS W/L L MM 5. NEXSEN JR.

ATTORNEY United States Patent DEVICE FOR FORMING REPLIQA IMAGES OFPARTICLE DISTRIBUTIONS IN A PLASMA STREAM Frederic H. Coensgen,Pleasanton, and William F. Cummins and William E. Nexsen, .lra,Liver-more, Calif., assignors to the United States of America asrepresented by the United States Atomic Energy (Iornmission Filed Feb.24, 1965, Ser. No. 435,161 5 Claims. (Cl. 258-715) ABSTRACT OF THEDISCLQSURE Device for forming visible replica images indicative ofinstantaneous particle distributions in a plasma stream, including asemi-transmissive element for attenuating the plasma stream, and havingan electrically polarized grid arranged to repel undesired chargedparticles from the attenuated stream and converting selected particlesimpinging thereon into a replica stream of electrons emerging therefrom.A scintillator screen is arranged to intercept portions of the replicaelectron stream, which are accelerated through light excluding means bymeans of an electrostatic field applied between said grid andscintillator to provide said replica image.

The present invention relates generally to devices for observing nuclearparticles and more particularly to a device for forming visible replicaimages indicative of the spatial distribution of streaming nuclearparticles termed a camera hereinafter, for simplicity.

As more is learned about the behavior of nuclear particles, thepotential benefits derivable from atomic energy is greatly increased.One of the characteristics of nuclear particles not completelyunderstood is particle interaction in the presence of electromagneticfields, for example, encountered in the generation and utilization ofnuclear particle beams and magnetically confined high temperatureplasmas. In confining plasmas, various types of instability problemshave been encountered which to a large extent have prevented the fullrealization of the potential of high temperature plasmas. As is wellknown, most of the instabilities result in the extremely rapid movementof the plasma across the magnetic confinin field to eventually encounterthe container walls and become destroyed. By observing the spatialdistribution of the plasma particles and the rapid movement of theplasma, the characteristics of the various types of instabilities can beinvestigated and studied. In the present art, there is usuallyinsufficient radiation from the plasma for conventional time-resolvedphotographic techniques such as those using a Kerr cell as a shutter.Furthermore, even in cases where the radiation is sufiicient,interpretation of the data often is complicated by intense radiationfrom sources other than the plasma under observation. Another difiicultyis encountered in pulse operated plasma systems and unstable plasmaswhere investigations are complicated in that the plasma distribution isnot reproducible. In such a case, a composite picture built up of datafrom several operations gives only a statistical average.

The present invention operates independently of the light or otherradiations from the plasma and provides a time resolved image whoseintensity is a function of the instantaneous plasma flux being viewed.As a brief description, the invention approaches the solution to theproblems set forth supra in the following manner. A grid placed in thepath of a stream of nuclear particles is selectively biased eitherpositively to repel positively ice charged particles and pass negativelycharged particles or neutrals, or biased negatively to repel negativelycharged particles and pass positively charged particles and neutrals.Where particles other than electrons are to be observed, it ispreferable to convert them to secondary electrons. These electrons,either secondary or primary, are distributed in facsimile to theparticle stream cross section. By electrostatically accelerating theseelectrons for example, through a light-impervious, highenergy-electronpervious sheet to impinge on an energeticparticle-to-light converter,e.g., plastic scintillator, an image is produced which is a replica ofthe stream crosssection. An ordinary light-sensitive camera can then beused to record the image. The light-impervious, highenergy-electronpervious sheet serves to eliminate the effects of any accompanyingundesirable radiation which would otherwise make it diificult if notimpossible to obtain and record the image of the distribution of theparticles of the nuclear particle stream.

The camera of the present invention further includes the feature of anextremely fast electronic shutter capable of shutter speeds in thenanosecond time range. The electronic shuttering action is obtained bycontrolling the electrostatic acceleration of the electrons which aredirected to impinge on the energetic-particle-to-light converter. As isknown, such conversion requires that the impinging particles have anenergy above a minimum threshold level. Rapid shuttering is obtained bymaintaining the electrostatic accelerating field at a level below thatrequired to impart sufficient energy to the electrons to initiate theconversion, and at a predetermined time increase the accelerating fieldfor a selected period to a level whereby the electrons are acceleratedabove the energy threshold level necessary for initiating theconversion. As a consequence of the rapid shutter characteristic of thepresent camera, an image representative of a substantially instantaneousdistribution of selected particles of a nuclear particle stream can beobtained.

Accordingly, it is an object of the present invention to provide adevice for observing the cross-sectional dis tribution of particles in astream of nuclear particles.

A further object of the present invention is to provide a replica of thedistribution of particles in a plasma stream, which replica is capableof being photographed.

Further, it is an object of the present invention to permit theapplication of time resolved photographic techniques to the study ofplasmas.

Still another object of the present invention is to obtain plasma streamcross-sectional particle distribution data which is not complicated byradiation from sources other than the plasma stream under observation.

Another object is to provide a time resolved image whose intensity is afunction of the instantaneous particle flux of the plasma being viewed.

It is yet another object of the present invention to provide a timeresolved replica of particle distribution in a plasma stream.

More particularly, it is an object of this invention to capture areplica of an instantaneous cross-section of a plasma stream whosecross-section varies with time and is not reproducible.

The means of achieving these and other objects will become more apparentfrom the following detailed description of a preferred embodiment of thepresent invention, taken in conjunction with the accompanying drawingwhich is a cross-sectional elevation view of one preferred embodiment ofthe present invention.

The plasma camera, i.e., image-forming device, of the present inventionis employed to selectively observe the spatial distribution of any ofthe various nuclear particles, e.g., positive and negative ions,electrons, or neutral particles, that may be found in a stream ofnuclear particles. The camera obtains an image of the spatialdistribution of the nuclear particles of a particle stream by linearlyconverting the energy of selected particles to light without alteringthe relative positions of the selected particles in the beam. The lightthus generated is an indication of the spatial distribution of theselected particles of the stream, and by observing or recording thislight, the particle distribution of the stream can be analyzed.

To accomplish this observation, the plasma camera of the presentinvention takes advantage of the coaction of the three principalsections comprising the camera: A particle selector, an image generator,and an image recorder. The particle selector is positioned to receive astream of nuclear particles and functions to selectively extracttherefrom the particles to be analyzed. In its simpliest form, theparticle selector accelerates and trans mits selected particles from theparticle stream to the image generator without altering the particlespositional relationship while simultaneously impeding the transmissionof any accompanying undesired particles. However, in the analysis ofhigh density particle streams, it is most beneficial to attenuate thedensity of the particle stream prior to generating an image reproductionof the selectively transmitted particles. As will be set forth in moredetail hereinafter, this can be accomplished by positioning asemi-pervious particle shield in front of the surface of the imageselector facing the impinging particle stream.

The selected particles are accelerated and directed to impinge on anenergelic-particle-to-light converter, i.e., image generator, whereinlight is emitted therefrom in proportionate relation to the number ofimpinging particles to form an image of the cross-sectional distributionof the particles in the nuclear particles stream. In those cases wherethe plasma camera is employed to observe selected particles in thepresence of extensive background light, e.g., particles originating froma plasma, the image generator may be shielded from the undesired lightby means of a suitable particle-pervious-light-impervious shield. Thelight emitted by the image converter is an accurate representation ofthe spatial distribution of the selected particles in the particlestream and may permanently be recorded by the simplest oflight-sensitive cameras.

As set forth hereinbefore, the present camera includes the feature of anextremely fast electronic shutter. Hence, in those cases Where it isdesired to observe all types of particles of a particle streamincluding, for example, both positively and negatively chargedparticles, the particle selector could be synchronously operated withthe shutter to obtain sequential observations of the various types ofparticles. More specifically, the particle selector would be operated topass first one type of particles than another. Simultaneously, theaccelerating electrostatic field would be adjusted to impart sufficientenergy to the selected particles whereby upon impingement upon the imagegenerator light is produced.

Although the plasma camera may talre the form of various embodiments inaccordance with the present invention, the specific embodimentcontemplated as best carrying out the invention is set forth in theimmediately following description and accompanying drawing. Suchspecific embodiment may be employed, for example, by being coupled atone end of an accelerator, fusion device such as a magnetic mirrormachine, etc., which is capable of delivering a charged particle orplasma beam.

In the figure, camera 11 is adapted to operate in an evacuatedatmosphere and is comprised of an electrically conducting particlesemi-transmissive plate 12 capping a first end 13 of an electricallyconductive housing 14. End 13 of housing 14 is adapted to allow camera11 to be mounted hermetically to a particle stream source (not shown). Aplastic scintillator sheet 16 or other suitableeuergetic-particle-to-light conversion means is mounted on transparentbacking plate 17 which in turn is secured to a mounting ring 18 securedwithin housing 14 spaced apart substantially parallel to plate 12. Alight-impervious high-energy-electron-pervious element 19 is disposedbetween scintillator 16 and plate 12. This may be a two thousandAngstrom aluminum coating insulatingly disposed on the surface 21 ofscintillator 16 facing plate 12. A grid 22 having a surface which iselectron emissivcly responsive to impinging particles, is mounted bystand-off insulators 23 between, parallel to, and insulatingly apartfrom scintillator 16 and plate 12. The second end 24 of housing 14 isprovided with mounting means 26 to receive appropriate light recordingmeans, e.g., a conventional light sensitive camera (not shown). Plate 12and housing 14 are electrically grounded and grid 22 is connected viawire 27 to a variable voltage source 28 having a range, e.g. of $300volts. Source 28 is referenced to housing 14 via wire 29. Metal film 19is electrically connected via conductor 31 to a high voltage pulsegenerator 32 which is referenced to housing 14 via wire 33.

in practice, inch Lucite has been used as the transparent backing plate17, Lucite being an acrylic resin, e.g., polymethyl methylacrylate,manufactured by E. I. du Pont de Nemours and Company. The scintillatorcan be polystyrene containing terphenyl or any of the other numerousscintillators used in high energy physics research.

The operation of camera 11 will be described with reference to its useto observe particles from a high temperature magnetically confinedplasma which includes both positively and negatively charged particles.A stream of plasma particles 34 originating from a high temperatureplasma moving in an evacuated atmosphere is directed to impinge onsemi-transmissive plate 12. This plate 12, comprised of a stack of threeTV color masks, passes approximately one-thousandth of the plasma intothe chamber between grid 22 and plate 12. Although quantitative figuresare unavailable, it has been observed in practice that voltage breakdownwill occur between grid 22 and sheet 19 if the plasma stream density isnot reduced utilizing a means such as plate 12. With a negative voltageon grid 22, the electrons in the plasma are repelled and the ions areaccelerated to impinge on the electron emission grid 22. Those ionsstriking the grid 22 release secondary electrons. With a positivevoltage on grid 22 the ions in the plasma are repelled and the secondaryelectrons therein are accelerated through the grid 22. In either case,electrons appearing at grid 22 are distributed in replica to thecross-sectional distribution of the plasma stream particles of thepolarity which are attracted to the grid.

A positive high voltage pulse, e.g., 20 kv. applied for 0.2-0.4microsecond, is imposed on film 19 to accelerate those electrons whichappear at grid 22 towards film 19. Note that this film 19 serves as alight shield and functions as a camera shutter which opens whensubjected to the high voltage positive pulse. This high voltage pulseimparts sufficient energy to some of these electrons emerging from grid22 so that they are able to penetrate metal film 19 and produce pointsource fluorescence in scintillator 16. In preferred practice, the imageproduced on the scintillator 16 may then be recorded by ordinaryphotographic techniques or may be sensed by other appropriate lightsensing means, e.g., an array of photocells. The most important singlefeature of this invention is the fact that this image has a crosssection which is substantially identical to the spatial distribution ofthose electrons or ions flowing in that portion of the plasma stream 34which is viewed by camera 11 a moment before the high voltage pulse isapplied to plate 19. It is to be noted that particle stream crosssection size that can be viewed by camera 11 is limited only by theeffective size of camera 11. The effective size of camera 11 isdetermined by diameter of unimpeded path from plate 12 to the cameraattached at the second end 24 of housing 14.

Although to date use of the camera 11 of the present invention has notbeen extended beyond observation of streams of charged particles, itconceivably may be used to observe streams of uncharged particles,provided a significant number of the particles possess energy sufiicientto cause secondary emission of electrons from grid 22. In such a case,grid 22 may be well placed at ground potential, there being norequirement to select particles of a particular polarity. However, ifstreams of charged particles are to be analyzed which include positivelyand negatively charged and neutral particles, at second grid having amesh selected to allow transmission of all particles would be interposedbetween grid 22 and plate 12. This second grid would serve as a firststage of selectivity with grid 22 serving as a second stage of selectionand electron generation. For example, to observe neutral particles froma stream including charged particles, the first grid would be chargednegatively to repel the negatively charged particles, and grid 22 wouldbe charged positively to repel the positively charged particles andallow only neutrals to impinge grid 22 and produce second-ary electrons.

In the figure, there are shown six circular metallic field aligningrings 36 insulated apart from each other and from grid 22. In practice,these rings 36 serve merely to provide a uniform electric field acrossthe face of the scintillator 16 to prevent image distortion. It is feltthat use of these rings 36 is well within the skill of the art and theiruse may be desirable where beam spreading problems are likely to beencountered.

As an aid to those who wish to practice the present invention, there isprovided below a table of relevant design values:

Outside diameter of housing 14 12 inches.

Effective camera diameter 9.5 inches.

Front plate 12 Stack of 3 TV color masks.

Front plate 12 transmission 0.72

Grid22 58% electron trans mission. 52-mesh stainless steel.

Number of field aligning rings 36 Field aligning ring 35 separation 0.25inch.

Front plate 12 to grid 22 separation 1 inch.

Grid 22 to scintillator 16 separation Do.

Depth of metallic coating material 19 2000 angstroms.

Grid 22 bias 300-+300 volts.

High voltage potential at accelerator electrode 19 20 kv.(approximately).

Optical camera S p e e d-Graphic Pacemaker with i n c h f5.6

Wollensak lens a n d Polaroid film back.

As this plasma camera has been used to date, the incident plasma densityhas not exceeded 5 X 10 ions/cm. With these ion densities, threeseventeen-inch TV masks with 0.01-inch holes, obtained from BuckbeeMears, Inc., St. Paul, Minn., have been found to work Well as theattenuating front plate 12.

It may be noted that the term plasma stream as used in this applicationincludes any stream of charged particles and is not necessarily limitedto those situations where there is a net zero charge, i.e., the positivecharges equal in number to the negative charges.

Although the invention has been described above in the spirit and scopeof the invention. For example, by providing an appropriateparticle-transmissive electrode in the position now occupied by thesemitransmissive plate 12, it may be possible to eliminate that plateand also the above-mentioned camera housing 14. Also, should there be notroublesome light effects accompanying the particle beam, it may bepossible to omit the light impervious metallic plate 19 from the surfaceof the scintillator 16. As a further modification, it may be consideredthat means other than a light sensitive camera may be used to senseand/or record the light image appearing on the above-mentionedscintillator 16. Furthermore, should the plasma stream contain particlesof a single polarity, it may be unnecessary to provide theabove-described grid means 22 to electrostatically select and rejectselected particles.

Hence, the scope of the present invention is intended to be limited onlyby the terms of the appended claims.

What is claimed is:

1. Device for forming visible image replicas indicative of the spatialdistribution of energetic particles in a plasma stream in an evacuatedregion, comprising, in combination:

(a) an elongated double-ended electrically conductive tubular housing incommunication with said evacuated region, and arranged for entry of saidplasma stream at one end for passage longitudinally therethrough;

(b) perforate semi-transmissive, electrically conductive plate meansdisposed across said first end of said housing to uniformly reduce andattenuate the particle density of said plasma stream passingtherethrough;

(c) a grid disposed in insulated, spaced relation in said housingtransversely across said attenuated plasma particle stream;

(d) a variable bipolar voltage source electrically connected betweensaid plate and said grid for applying an electrostatic potentialtherebetween to repel particles of undesired polarity and to select andaccelerate particles of the desired polarity from said attenuated plasmaparticle stream to impinge upon one side and cause emission of acorresponding replica stream of secondary electrons from the second sideof said grid;

(e) sheet phosphor means disposed in spaced relation to the second sideof said grid in said housing;

(f) second electrode means disposed across said tubular housing betweensaid grid means and phosphor means, said second electrode means beingpervious to high energy electrons and impervious to light; and

(g) voltage supply means connected between said grid and said secondelectrode means to apply an electrical pulse thereto for acceleratingsaid secondary electrons to a sutficiently high energy level topenetrate said second electrode means and impinge upon said phosphormeans.

2. Apparatus according to claim 1 for sampling high density plasmaparticle beams, further defined in that said first pervious electronmeans has a porosity sufiicient to attenuate said beam to less thanabout one one-thousandth of the original density thereof.

3. Apparatus according to claim 1, further defined in that voltagesupply means is a voltage pulse generator for impressing upon said gridand secondary electrode means a voltage pulse of fractional microsecondduration to accelerate said secondary electrons during a preselectedtime interval exclusively.

4. Apparatus according to claim 3, wherein said phosphor means comprisesa sheet scintillator plastic material, said second electrode means is athin metallic film deposited on the first side of said sheetscintillator material, and a transparent backing plate is disposed atthe second side of said sheet scintillator material in capping relationto said housing.

7 8 5. Apparatus according to claim 4, wherein said per- 3,002,1019/1961 Anderson et a1. 250-213 forate semitransmissive plate meanscomprises three 3,012,149 12/1961 Heimann et a1. 250-213 superposedcolor television masks with 0.01-inch holes. 3,107,303 10/1963 Berkowitz250-213 3,121,796 2/1964 Reed 250-213 References Cited 5 UNITED STATESPATENTS RALPH G. NILSON, Primary Examiner. 2 421,132 5 1947 Bay e2502.213 ARCHIE R. BORCHELT, Examiner. 2,572,494 10/1951 Krieger et a1.250-213 AU Assistant 2,903,596 9/1959 Reed 250-213

